1. Field of the Invention
The present invention relates to plasma arc cutting technology, and in particular relates to an improved method of use of secondary gas, with the object of improving the bevel angle of the cut faces.
2. Description of the Related Art
In U.S. Pat. No. 2,689,310, adjustment of the bevel angle of the cut surfaces is made possible by means of the strength of rotation of a secondary gas rotary flow, by supplying a secondary gas rotary flow that rotates in the same direction at the circumference of the rotating plasma arc. By the way "bevel angle" is meant the angle made by the cut surface with the surface perpendicular to the work under-surface. In general, in plasma cutting, as shown in FIG. 1(A), the cross-sectional shape of the kerf (cut groove) 5 of a work 3 has a tapered shape which becomes narrower at greater depth (i.e. the upper kerf width is wider and the lower kerf width is narrower), due to pinching of the plasma arc 1. Consequently, the cut surfaces 7L and 7R on both sides are not perpendicular (bevel angle 0 degrees) with respect to the work under-surface 9, but tapered by a few degrees. The degree of this taper increases as the cutting speed is made higher. However, if a secondary gas rotary flow is supplied in the same rotary direction at the circumference of the plasma arc rotary flow, as shown in FIG. 1(B), when the plasma arc 1 reaches work 3, the central axis 1A of plasma arc 1 is bent over and inclined in the right or left direction towards the cutting direction (i.e. the direction going from in front of the plane of the drawing in FIG. 1 to behind the plane of the drawing) (whether to right or left is determined by the direction of rotation; in the example of FIG. 1, the arc 1 is inclined to the right due to the rightwards rotary direction seen from above). Consequently, at the cut surface 7R on the side where arc axis 1A is inclined, the bevel angle is corrected towards the 0 degrees direction. When the strength of the secondary gas rotary flow (i.e. the secondary gas flow rate) is increased, the angle of inclination of the arc axis becomes correspondingly larger, so it is possible not only, of course, to correct the bevel angle to 0 degrees but also to perform over-correction further to the opposite side beyond 0 degrees. Consequently, by setting the strength of the secondary gas rotary flow (secondary gas flow rate) to a suitable value, the bevel angle can be adjusted to a desired value (typically, 0 degrees).
In this specification, this technique is termed the "double swirl technique", meaning a "double rotary flow of the plasma arc and secondary gas". By means of the double swirl technique, cutting at high speed can be achieved, thus enabling the productivity of plasma cutting devices to be raised.
In general in a plasma cutting device, cutting proceeds while a plasma torch is displaced with respect to the work along a cutting line matching the shape of the product that is to be cut out, by means of a displacement system whereby the plasma torch is held and displaced in the XY directions. In this process, while high-speed cutting is performed in regions of the cutting line which are straight lines or gentle curves, in the region of corners or sharp curves, or holes, in particular holes of small diameter, cutting is performed at low speed. The reason for this is that, since the direction of displacement changes abruptly at corners, sharp curves or holes etc., the XY displacement system is unable to track this at high speed, so there is a deterioration of accuracy of the cutting track.
The degree of taper of the cut surfaces illustrated in FIG. 1(A) is large during high-speed cutting but small during low-speed cutting. This therefore gives rise to the problem that, if it is assumed that, in plasma cutting in which the double swirl technique is adopted, the intensity of the secondary gas rotary flow (secondary gas flow rate) is set such that the bevel angle is exactly 0 degrees during high-speed cutting of straight lines or gentle curves of the cutting line, when cutting of corners or sharp curves is performed at low speed, the bevel angle is over-corrected and is therefore not 0 degrees.
Also, as shown in FIG. 2, there is the phenomenon that, at the leading face 11 of cutting of work 3, cutting at the bottom lags behind cutting at the top (hereinafter, the distance 13 representing the amount of the lag of cutting at the lower edge with respect to the upper edge of the leading cutting face 11 is termed the "cutting lag"). Consequently, in particular at locations where the direction of cutting changes considerably such as at corners or sharp curves, a positional offset is produced between the track on the upper side of kerf 5 (hereinafter this is called the "upper kerf track") 15 and the track on the lower side thereof (hereinafter this is called the "lower kerf track") 17. For example, FIG. 3 illustrates a plan view of a work 3 in the vicinity of a corner 19A of a cutting line 19. Although the upper kerf track 15 (shown by the solid line) faithfully corresponds to the displacement of the plasma torch, cutting out an accurate corner 19A having a sharp edge, the lower kerf track 17 (shown by the broken line for ease of visibility) takes a short-cut on the inside of corner 19A and so cannot cut a sharp edge because of cutting lag. Thus, since positional offset between the upper and lower kerf tracks 15 and 17 is produced by the cutting lag, not only does the bevel angle change, but also the accuracy of the cutting track (in particular that of the lower kerf track 17) is adversely affected.